1. Field of the Invention
The present invention relates to a process for producing a novel reinforced composition having an excellent productivity and workability and capable of providing a vulcanizate having an excellent strength and modulus, and also to a vulcanizate obtained therefrom.
2. Description of the Related Art
Reinforced rubber compositions used in tires have heretofore been prepared by incorporating short fibers made of, for example, nylon (i.e., polyamide), polyester, or formalized polyvinyl alcohol, into vulcanizable rubber. The reinforced rubber compositions thus produced, however, have a disadvantage in that the strength and modulus of the vulcanizates derived therefrom, especially at high elongation, are not sufficiently high. This is because the diameter of the fibers is large and no bonding occurs between the fibers and the rubber. Thus, there is a demand for the development of reinforced rubber compositions capable of providing vulcanizates having a higher strength and modulus.
Under these circumstances, Japanese Unexamined Patent Publication (Kokai) No. 53-8682 proposes both a process for reinforcing elastomer and a reinforced elastomer composition. According to this proposal, a fiber-forming polymer is mixed with rubber followed by extrusion of the resultant mixture. The reinforced rubber composition thus obtained, however, has the disadvantages of low tensile strength, low modulus at a high elongation, and low peeling strength which exhibits an adhesive force with other materials, of the vulcanizates thereof because fibers having a large diameter and film are formed and because no bonding occurs between the rubber and polymer (i.e., fiber) at the interface of the fibers and the rubber. Thus, a fiber-reinforced rubber composition suitable for use as parts of a tire cannot be obtained according to this proposal.
To improve conventional reinforced rubber compositions, the present inventors proposed a process for producing fiber-reinforced rubber compositions, in Japanese Patent Application No. 56-117044. According to this process, 100 parts by weight of vulcanizable rubber, 1 to 70 parts by weight of nylon having a number-average molecular weight of less than 200,000, and 0.2 to 2.5 parts by weight, based on 100 parts by weight of the total amount of the rubber and the thermoplastic polymer, of a resol type alkylphenol formaldehyde resin precondensate are mixed together at a temperature of not less than the melting point of the thermoplastic polymer, but not more than 270.degree. C., the mixture is extruded from an extruder at a temperature of not less than the melting point of the thermoplastic polymer, but not more than 270.degree. C., and the extrudate is drawn at a temperature of less than the melting point of the thermoplastic polymer.
However, this production process still involves practical disadvantages. For example, since a resol type alkylphenol resin capable of reacting with the rubber upon heating is used, the control of the graft reaction between the nylon and the rubber under heating is difficult. Also, the rubber usable in this process is limited to natural rubber, and nylon fibers having a large diameter are sometimes included in the rubber. Furthermore, when the amount of the nylon is more than 70 parts by weight based on 100 parts by weight of the rubber, the nylon is included in the form of film in the rubber. Thus, a reinforced rubber composition having only a small nylon content can be obtained and, therefore, the rubber product obtained by using, as a masterbatch composition, this reinforced rubber composition becomes expensive and does not have sufficiently satisfactory quality.